1. Field of the Invention
The present invention relates to an optical fiber unit which is suitable for use in a laying method of a fluid pressure-sending system such as an air blown fiber method or other such systems.
2. Description of the Prior Art
The air blown fiber method, a new method for laying a communication line material, is disclosed, for example, in Japanese Patent Unexamined Publication No. Sho-59-104607. In such a method, a pressure-sending device 31 as shown in FIG. 5 is used. In this device, a communication line material is pressure-sent and advanced forward into a previously laid duct line 32 by means of an air flow so as to be laid in duct line 32. In FIG. 5, the reference numeral 34 designates a feed head to be attached to an end portion of duct line 32, and reference numerals 35 and 36 are driving wheels for sending communication line material 33 into duct line 32. Numeral 37 illustrates an air leading inlet for leading air for pressure-sending the communication line material 33. An air seal 38 prevents air from leaking through an inlet portion for communication line material 33 in feed head 34. Air seal 38 is provided because duct line 32 is so long that pressure loss is large.
It is necessary that communication line material 33 used in such a method is made lightweight with a large surface area so as to be suitable for pressure-sending by means of an air flow. Conventionally, communication line material, (an optical fiber unit) to be laid according to the above-described method, is optical fiber units as shown in FIGS. 6 and 7 have been used.
Optical fiber unit 40 shown in FIG. 6 is composed of seven optical fibers 41 which are bundled with primary coating 42 made of polypropylene. Primary coating 42 is further coated with a porous layer 43 made of foam polyethylene. The optical fiber unit is reduced in weight and increased in surface area due to the provision of porous layer 43.
Optical fiber unit 50 shown in FIG. 7 is composed of four sub-units 53 each of which is constituted by a pair of optical fibers 51 coated with a primary coating 52. Sub-units 53 are arranged on the circumference of interposition quadrat 54. A secondary coating 55 is applied on the periphery of sub-units 53 and the interposition quadrat 54, and a porous layer 56 that reduces the weight of optical fiber unit 50 and increases the surface area of fiber unit is then applied on the secondary coating 55.
As described above, in conventional optical fiber units 40 and 50 applied to the air blown fiber method, the number of optical fiber cores was small (the number of optical fiber cores being seven or eight), and the optical fibers were gathered in a straight state. Accordingly, these optical fiber units have the following advantages.
(1) Since the number of optical fiber cores is small, there is little distortion of optical fibers even if they are laid in a curved duct line.
(2) Since optical fibers were gathered in a straight state, the drag acting in the longitudinal direction is large. Accordingly, the communication line material 33 can be sent smoothly by the large drag when the communication line material 33 (for example, the optical fiber unit 40 or 50) is sent into the high-pressure duct line 32 while being pinched by the driving wheels 35 and 36 as shown in FIG. 5.
(3) Since optical fibers are arranged straight, manufacturing is relatively simple.
Because the quantity of information transmission has recently increased, it is desirable to enlarge the capacity of optical fiber units such as those described above. That is, it is desirable to maximize the number of optical cores contained in an optical fiber unit. However, when a straight-arranged optical fiber unit, in which the number of optical fiber cores is large (not less than nine), is manufactured, the following problems occur.
(1) Since the number of optical fiber cores is large, distortion of arrangement is caused.
(2) As the number of cores is increased so that the diameter of the optical fiber unit becomes large, a negative influence of distortion due to bending of the optical fiber unit occurs. This problem is remarkable particularly in the state in which transmission loss increases at a time of low temperature. Such an increase of transmission loss at a time of low temperature is caused by shrinkage of the outer porous layer concentrated only in part on the number of optical fibers. Accordingly, the transmission characteristic becomes different depending on the position of arrangement of the optical fibers, thereby making it impossible to attain homogeneous transmission.
(3) Since the optical fiber unit becomes high in rigidity and even though the drag required in sending the optical fiber unit is sufficient, there is a possibility that the optical fiber unit is difficult to bend along a curve of a duct line having a curved portion. This makes it difficult to lay the optical fiber unit in a previously laid duct line.